16 



Usinger: Introduction 



Spring Overturn 



Summer Thermal 

 Stratification 



> Fall Overturn 



Intro, fig. 18. Diagrams showing thermal stratification and mixing in a temperate lake of the 

 second order during the four principal seasons. Temperatures ore given in degrees centigrade 

 (4 C. =39.2 F.; 22 C.=71.6 F.) Figures for the thermocline were chosen arbitrarily to illustrate 

 the general principle (Welch, 1952). 



upper zone of summer circulation by wind (epilimnion) 

 with relatively uniform temperatures, an intermediate 

 zone (thermocline) with rapid fall in temperature (Birge 

 Rule — 1° C or more per meter), and a lower zone of 

 summer stagnation (hypolimnion). Very deep lakes 

 such as Lake Tahoe and shallow lakes in the tropics 

 never freeze because winter temperatures are not low 

 enough for a sufficiently long period to cool the 

 entire mass of water below 4°C. Ponds may have no 

 thermocline or hypolimnion because wind action 

 mixes the shallow water from top to bottom. 



Lake classification. — As in the lotic or running 

 waters, so also in the lentic or standing waters, 

 classification is difficult. Lakes of infinite variety 

 merge into ponds and thence into swamps or bogs, 

 and lines of demarcation between types are neces- 

 sarily arbitrary. In spite of inherent difficulties, 

 limnologists have devoted far more time and thought 

 to lake classification and to lakes in general than 

 to streams, probably because lakes are more stable 

 and are easier to measure physically and sample 

 quantitatively. Among the criteria used for the classi- 

 fication of lakes, temperature and depth were selected 

 by Forel (1892-1904), productivity was chosen by 

 Thienemann (1926), and source of water, nature of 

 substrate, size, and other criteria have been variously 

 used. None of these has proved to be satisfactory 

 but Forel's thermal classification is theoretically 

 useful, and Thienemann's trophic classification has 

 gained universal acceptance and is used in common 

 parlance without precise limits. 



In the thermal classification (intro. fig. 19), lakes 

 are arranged according to "Types," based on surface 

 temperatures, and "Orders," based on bottom tempera- 

 tures. Three types are recognized: the Polar Type, 

 with surface temperatures never above that of the 

 maximum density of water (4°C, 39.2°F); the Tem- 

 perate Type, with surface temperatures above 39.2°F 

 in summer and below 39.2°F in winter; and the Trop- 

 ical Type, with surface temperatures never below 

 39.2°. Each of these types is then placed as the First 

 Order, deep lakes (approximately 200 feet or more) 

 with bottom temperatures constant near the point of 

 maximum density (39.2°); or the Second Order, inter- 

 mediate lakes (approximately 25 to 200 feet) with 



bottom temperatures that fluctuate near 39.2°; or the 

 Third Order, shallow lakes (approximately 25 feet or 

 less) with bottom temperatures at or near those of 

 the surface. This classification, though ill-defined 

 as to depth and misleading in terminology, is based 

 on sound physical principles. In general, California 

 lakes at middle and high elevations are of the tem- 

 perate type, and lowland lakes are of the tropical 

 type. However, Lake Tahoe, at 6,000 feet elevation, 

 is so deep that it does not freeze. Therefore it would 

 be classed as a tropical lake of the first order. This 

 clearly demonstrates that other criteria must be used 

 if a classification is to be meaningful in a biological 

 sense. 



Thienemann's classification is more useful in this 

 sense. It is an established fact that productivity is 

 dependent in a general way on the depth and stage of 

 succession of a lake. Thus in deep lakes of recent 

 origin the bottom is likely to be lacking in organic 

 nutrients, the biota is not rich, and the volume of 

 water below the level of effective penetration of 

 light (heterotrophic or decomposition zone) exceeds 

 that of the producing zone (autotrophic or photosyn- 

 thetic zone). Such lakes are called Oligotrophic and 



POLAR TYPE 



TEMPERATE TYPE 



FIRST ORDER 

 POLAR TYPE 



SECOND ORDER 

 POLAR TYPE 



THIRD ORDER 



Intro, fig. 19. Thermal classification of lakes arranged accord- 

 ing to "types," based on surface temperatures above or below 

 that of the maximum density of woter (39.2 F), and "orders," 

 based on bottom temperatures above or below 39.2 . First order- 

 lakes are deepest, second order intermediate, and third order 

 shallow (Whipple, 1927). 



